1. Field of the Invention
The present invention relates to a self-supporting cable which is installed overhead and is used as a subscriber line, and also relates to a method for manufacturing the self-supporting cable.
2. Description of the Background Art
Self-supporting cables having a construction as shown in FIGS. 8A to 8C have been suggested as optical fiber cables to be installed overhead for subscriber lines in optical communications networks. FIGS. 8A and 8B show an example of such a conventional self-supporting cable, wherein FIG. 8A is a perspective view thereof and FIG. 8B is a cross-sectional view thereof. FIG. 8C is a cross-sectional view of a cable core used in the self-supporting cable. In FIGS. 8A to 8C, reference numeral 41 denotes optical fiber ribbons, 41a denotes optical fibers, 41b denotes optical fiber coatings, 41c denotes coated optical fibers, 41d denotes ribbon coatings, 42 denotes a cable core, 43 denotes a support member, 44 denotes strength members, 45 denotes an outer cover layer, 46 denotes supporting portion, 47 denotes connecting portions, 48 denotes a main cable portion, and 49 denotes window portions.
The coated optical fibers 41c are constructed by applying the optical fiber coatings 41b on the optical fibers 41a, and the optical fiber ribbons 41 are constructed by arranging a plurality of coated optical fibers 41c in parallel and applying the ribbon coating 41d thereover. The cable core 42 is constructed by stacking a plurality of optical fiber ribbons 41. The stack of optical fiber ribbons may be wound by a tape or a string so as to maintain the stack. In addition, such stack may also be twisted in the longitudinal direction thereof.
The cable core 42, the support member 43, and two strength members 44 are arranged in parallel to each other in the longitudinal direction, and the outer cover layer 45 is formed by extruding a thermoplastic resin by using an extruder such that it has a cross-section of figure-8 shape so as to cover these members. The support member 43 is formed of stranded steel wires, FRP, etc., and the strength members 44 are formed of a steel wire, etc. To improve the flexibility of the self-supporting cable, the support member 43 and the strength members 44 are arranged such that the central axis thereof are on the same plane. The support member 43 serves to support the self-supporting cable when it is installed overhead. In addition, the strength members 44 serve to prevent the outer cover layer 45 from shrinking as the internal strain generated during the extrusion process reduces over time.
In the above-described self-supporting cable, the cable core 42 is constructed such that there is an excess length therein, so that, even when the self-supporting cable is installed overhead and tension is applied thereto, the cable core 42 is not subject to the tension. The supporting portion 46 includes the support member 43 and a part of the outer cover layer 45 around the support member 43, and the main cable portion 48 includes the cable core 42 and a part of the outer cover layer 45 around the cable core 42. The connecting portions 47, which are also a part of the figure-8 shaped outer cover layer 45, connect the supporting portion 46 and the main cable portion 48 with intervals in the longitudinal direction.
The connecting portions 47 are formed by intermittently removing portions of the connecting portion, which was continuous in the longitudinal direction upon the formation of the outer cover layer 45, by using an automatic cutter, such that the window portions 49 are formed between the connecting portions 47.
In the process of forming the outer cover layer 45, the feed rate at which the cable core 42 is fed to a crosshead of the extruder is set higher than the feed rate at which the support member 43 is fed thereto. Thus, the main cable portion 48 is made longer than the supporting portion 46 so there is an allowance in the length thereof. Since the feed rate of the cable core 42 and that of the support member 43 different, the resin forming the outer cover layer 45 is subject to an unbalanced strain in the extrusion process.
Accordingly, it is difficult to form the outer cover layer 45 so that it has a smooth figure-8 shape. In particular, when a clearance is provided between the cable core 42 and the outer cover layer 45 in the main cable portion 48, the outer cover layer 45 may not have a concentric shape around the cable core 42; the outer cover layer 45 may be deformed as though it is compressed.
FIG. 9 is a cross-sectional view which shows an example in which a clearance is provided between the cable core 42 and an outer cover layer 45′ in a main cable portion 48′. As shown in FIG. 9, in the main cable portion 48′, the outer cover layer 45′ does not have a concentric shape and inward protrusions are formed on the inner wall thereof at positions close to the strength members 44. When the outer cover layer 45′ is thus deformed, the protrusions formed on the inner walls may abut against and press the cable core 42. Thus, transmission characteristics of optical fibers contained in the core cable 42 may be degraded.